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From Robert Samuel Newson <rnew...@apache.org>
Subject [DISCUSS] Rebase CouchDB on top of FoundationDB
Date Wed, 23 Jan 2019 10:00:15 GMT
Hi,

CouchDB 2.0 introduced clustering; the ability to scale a single database across multiple
nodes, increasing both the maximum size of a database and adding native fault-tolerance. This
welcome and considerable step forward was not without its trade-offs. In the years since 2.0
was released, users frequently encounter the following issues as a direct consequence of the
2.0 clustering approach:

1. Conflict revisions can be created on normal concurrent updates issued to a single database,
since each replica of a database shard independently chooses whether to accept a given update,
and all replicas will eventually propagate updates that any one of them has chosen to accept.
2. Secondary indexes ("views") do not scale the same way as document lookups, as they are
sharded by doc id, not emitted view key (thus forcing a consultation of all shard ranges for
each query).
3. The changes feed is no longer totally ordered and, worse, could replay earlier changes
in the event of a node failure (even a temporary one).

The idea is to use FoundationDB as the new CouchDB foundational layer, letting it take care
of data storage and placement. An introduction to FoundationDB would take up too much space
here so I will summarise it as a highly scalable ordered key-value store with transactional
semantics, provides strong consistency, scaling from a single node to many. It is licensed
under the ASLv2 but is not an Apache project.

By using FoundationDB we can solve all three of the problems listed above and deliver semantics
much closer to CouchDB 1.x's behaviour while improving upon the scalability advantages that
2.0 introduced. The essential character of CouchDB would be preserved (MVCC for documents,
replication between CouchDB databases) but the underlying plumbing would change significantly.
In addition, this new foundation will allow us to add long wished-for features more easily.
For example, multi-document transactions become possible, as does efficient field-level reading
and writing. A further thought is the ability to update views transactionally with the database
update.

For those familiar with the CouchDB 2.0 architecture, the proposal is, in effect, to change
all the functions in fabric.erl so that they work against a (possibly remote) FoundationDB
cluster instead of the current implementation of calling into the original CouchDB 1.x code
(couch_btree, couch_file, etc).

This is a large change and, for full disclosure, the IBM Cloudant team are proposing it. We
have done our due diligence in investigating FoundationDB as well as detailed investigation
into how CouchDB semantics would be built on top of FoundationDB. Any and all decisions on
that must take place here on the CouchDB developer mailing list, of course, but we are confident
that this is feasible.
During those investigations we have identified a small number of CouchDB features that we
do not yet see a way to do on FoundationDB, the main one being custom (Javascript) reduces.
This is a direct consequence of no longer rolling our own persistence layer (couch_btree and
friends) and would likely apply to any alternative technology. 

I think this would be a great advance for CouchDB, preserving what makes CouchDB special but
taking advantage of the superbly engineered FoundationDB software at the bottom of the stack.

Regards,
Robert Newson
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